The invention relates to a method for adjusting the amount of air in a closed air supply system for a chassis, in which the pressure is measured in a collector line which connects all the air spring units to one another, and the actual compressed-air amount of the air supply system is calculated therefrom with the inclusion of further known parameters, is compared with a target compressed-air amount and the differential compressed-air amount is equalized accordingly. Air supply systems of this type are used, for example, for the ride height regulation of motor vehicles.
An air supply system of this type is known from EP 1 243 447 A2. According to its FIG. 2, said air supply system comprises substantially an air supply unit and a plurality of air spring units for supporting the vehicle superstructure. A compressor and an air pressure accumulator belong to the air supply unit mainly. Here, the compressor has a first suction line having a connection to atmosphere and a first pressure line which leads to the air pressure accumulator via an air dryer and a first 3/2-way valve. The compressor therefore supplies the air pressure accumulator with air from atmosphere. In addition, the compressor has a connection to the air spring units via a second suction line and via a second 3/2-way valve. The compressor fills compressed air from the air spring units into the air pressure accumulator via said second suction line and via its first pressure line. The compressor has a connection to the air pressure accumulator via a third suction line and via the first 3/2-way valve, and has a connection to the air spring units via a second pressure line and via the second 3/2-way valve. As a result, compressed air is conveyed out of the air pressure accumulator into the air spring units.
The air spring units are arranged parallel to one another, each air spring unit being assigned a 2/2-way valve and all 2/2-way valve units being connected to the air supply unit via a collector line. A pressure sensor is situated in the common collector line of the directional valve unit.
Closed air supply systems of this type operate within a predetermined performance range, the limits of which can be undershot by compressed air escaping through a leak or the pressure volume being reduced by a temperature drop, or the limits of which can be exceeded by the pressure volume being increased by a temperature rise. Within the ride height regulation of the vehicle, this has the effects that raising of the vehicle superstructure is slowed down if the pressure volume is too small, and that lowering of the vehicle superstructure is slowed down if the pressure volume is too high.
In order to ensure the performance range within its permissible limits, the pressure in the air spring units and in the accumulator has to be checked constantly, in order to refill compressed air into the air pressure accumulator in time or to discharge compressed air out of the air pressure accumulator. The performance range is monitored by a regular measurement of the pressure in the air spring units and in the air pressure accumulator in order to determine the overall pressure volume in the air supply system. For pressure measurement in the air spring units, the corresponding 2/2-way valves are opened one after another in the event of a corresponding shut-off position of the two 3/2-way valves in the supply unit, and the pressures which are set in the collector line in the process one after another are measured with the aid of the pressure sensor. For pressure measurement in the air pressure accumulator, flow is applied to the corresponding 3/2-way valves, in order that the pressure from the air pressure accumulator prevails at the pressure sensor. The requirement or the excess of compressed air volume is calculated from the pressures which are measured in the collector line for the air spring units and the air pressure accumulator, and the corresponding switching positions are implemented on the 3/2-way valves, in order to equalize the compressed-air amounts in the air pressure accumulator.
However, this method has disadvantages. For instance, if the 2/2-way valves are opened, pressure volume equalization occurs between the air spring units on one side and the collector line including the dryer and compressor volume on the other side on account of different pressures. This delays the pressure measurement, because the pressure in the collector line has to be equalized with the pressure in the air spring units before the measurement. Moreover, a change in the height of the air spring units and therefore of the vehicle superstructure necessarily occurs as a result of the pressure volume equalization. Undesired changes of this type in the vehicle height during the pressure measurement and during the valve switching are a safety risk, especially as the magnitude of the height changes cannot be estimated in advance. Here, the safety risk rises as the load becomes greater.
The invention is therefore based on the object of developing a method of the generic type, with which possible pressure and volume equalization operations via the switchable valves are avoided as far as possible.